1. Field of the Invention
This invention relates to thread wound golf balls. More specifically, it relates to thread wound golf balls having large centers that provide improved manufacturability and playability characteristics.
2. Description of the Related Art
Until the late 1960's, most golf balls were constructed with a thread wound core and a cover of compounds based on natural (balata and gutta percha) or synthetic transpolyisoprene. These golf balls have been and are still known to provide acceptable flight distance. Additionally, due to the relative softness of the balata cover, skilled golfers are able to impart various spins on the ball in order to control the ball's flight path (e.g., "fade" or "draw") and "bite" characteristics upon landing on a green.
With the advent of new materials developed through advances and experimentation in polymer chemistry, two-piece golf balls have been developed. The primary difference between a two-piece golf ball and a three-piece golf ball is the elimination of the rubber thread windings found in the three-piece balls. A relatively large center in a two-piece ball takes the place of the relatively small center and thread windings of a three-piece ball having the same overall diameter.
The two-piece balls have proven to be superior to three-piece balls in the areas of distance and durability. With respect to durability, the balls have proven to be more durable when repeatedly struck with golf clubs and more durable when exposed to a variety of environmental conditions such as high heat when stored, for example, in an automobile trunk. However, two-piece balls are, in general, considered to have inferior characteristics of feel and workability when compared to three-piece balls.
Attempts have been made to blend the characteristics of a three-piece ball with those of a two-piece ball. For example, a three-piece golf ball having a relatively large center is disclosed in UK Patent Application No. GB 2 307 865 A to Sano. In this UK patent application, a thread wound golf ball is disclosed having a center made of a vulcanized rubber composition with a diameter of between 30 and 38 millimeters. The center exhibits a deformation amount of between 1.2-2.5 millimeters or 0. 047-0.098 inches when a load of between 10 Kg initial and 30 Kg final is applied. The cover is composed of an ionomer resin, a maleic anhydride-modified thermoplastic resin and an epoxidated thermoplastic resin having a JIS-A hardness of from 30 to 90.
Although exhibiting some favorable characteristics of two-piece and three-piece balls, the center's deformation amount is too high to provide the launch characteristics of a true two-piece ball. Furthermore, the ionomer-based cover material, as is well known, will not produce the same playability characteristics of a three-piece ball with a balata cover even though the cover should exhibit superior durability characteristics.
Another attempt at blending the characteristics of two-piece and three piece balls is described in European Patent Application 0 674 923 A1 to Kato. In the Kato application, a wound golf ball is disclosed having a center composed of a vulcanized rubber composition having a diameter of between 30 and 35 millimeters. The center is described as having a deformation amount of between 1.2-2.5 millimeters or 0.047-0.098 inches when a load of between 10 Kg initial and 30 Kg final is applied.
Like the Sano reference, the Kato wound ball's center's deformation amount is too high to provide the launch characteristics of a true two-piece ball. Furthermore, the higher part of the described diameter range is not sufficient to impart the favorable characteristics of a true two-piece ball.
A still further attempt to combine the best characteristics of two-piece and three-piece balls is set forth in U.K. Patent Application No. GB 2 301 779 A to Maruko. The Maruko reference discloses a thread-wound golf ball wherein the center has a diameter of between 30 to 35 millimeters and a weight of between 19.5 to 29.0 g. The golf ball is described as having a deformation amount of between 2.5-3.7 millimeters or 0.098-0.146 inches when subjected to a 100 Kg load. The center has an intrinsic frequency of from 2,000-4,000 Hz when dropped 120 cm onto a steel plate. This golf ball design suffers from the same set backs as the previously described golf balls.
The primary problem with all the recited golf balls is that the centers are not large enough to move the weight to the periphery to provide sufficient moment of inertia to maximize distance without compromising ball velocity. To maintain ball velocity while increasing distance travel, the material used out of necessity must have high resilience. The golf balls disclosed do not have this combination of a large center made with materials exhibiting high resilience characteristics.
It has now been discovered that a relatively large, heavy center can be constructed of high resilience materials to increase the moment of inertia of a golf ball to increase distance travel. In combination with the other golf ball components, i.e., thread windings and an appropriately selected cover, distance maximization can be achieved without compromising ball initial velocity, PGA compression or shot feel and control.
Apart from the center composition and size, one of the key components to the invention is the material used for the cover. For many years, balata-based compounds were the material of choice to construct golf ball covers. Though possessing many desirable properties such as being readily adaptable to molding, there are substantial drawbacks to use of balata or transpolyisoprene-based compounds for golf ball covers. From a manufacturing standpoint, balata-type materials are expensive and the manufacturing procedures used are time consuming and labor-intensive, thereby adding to the material expense. From a player's perspective, golf balls constructed with balata-based covers are very susceptible to being cut from mishits and being sheared from "sharp" grooves on a club face. As a result, they have a relatively short life span.
In response to these drawbacks to balata-based golf ball covers, the golf ball manufacturing industry has shifted to the use of synthetic thermoplastic materials, most notably ionomers sold by E. I. DuPont De Nemours & Company under the name SURLYNO.RTM..
Thread wound balls with ionomer covers are less costly to manufacture than balls with balata covers. They are more durable and produce satisfactory flight distance. However, these materials are relatively hard compared to balata and thus lack the "click" and "feel" of a balata covered golf ball. "Click" is the sound emitted from the impact of a golf club head on a golf ball. "Feel" is the overall sensation transmitted to the golfer through the golf club after striking a golf ball.
In an attempt to overcome the negative factors of the hard ionomer covers, DuPont introduced low modulus SURLYN.RTM. ionomers in the early 1980's. These SURLYN ionomers have a flexural modulus of from about 3000 to about 7000 PSI and hardness of from 25 to about 40 as measured on the Shore D scale--ASTM 2240. The low modulus ionomers are terpolymers, typically of ethylene, methacrylic acid and n- or iso-butylacrylate, neutralized with sodium, zinc, magnesium or lithium cations. E. I. DuPont De Nemours & Company has disclosed that the low modulus ionomers can be blended with other grades of previously commercialized ionomers of high flexural modulus from about 30,000 to 55,000 PSI to produce balata-like properties. However, "soft" blends, typically 52 Shore D and lower (balata-like hardness), are still prone to cut and shear damage.
The low modulus ionomers when used without blends, produce covers with very similar physical properties to those of balata, including poor cut and shear resistance. Worse, wound balls with these covers tend to go "out-of-round" quicker than wound balls with balata covers. Blending with hard SURLYN ionomers was found to improve these properties. It has been found that use of any of the balata or ionomer-based cover compounds do not provide the desired combination of superior playability, durability and amalgamation characteristics when combined with the large center of the present invention even though these cover materials can be used with the invention.
Another approach taken to provide a golf ball cover that has the playing characteristics of balata without the inferior durability characteristics is described in U.S. Pat. No. 5,334,673 ("the '673 patent") assigned to the Acushnet Company. The '673 patent discloses a urethane cover composition comprising a diisocyanate, a polyol and a slow-reacting polyamine curing agent. The diisocyanates claimed in the '673 patent are relatively fast reacting. Due to this fact, catalysts are not needed to lower the activation energy threshold. However, since relatively fast-reacting prepolymer systems are used, the reaction rate cannot be easily controlled thereby requiring the implementation of substantial processing controls and precise reactant concentrations in order to obtain a desired product.
To avoid the problems associated with fast-reacting prepolymer systems, slow-reacting systems such as Toluene diisocyanate (TDI) prepolymer systems can be employed. However, these systems, while avoiding the problems associated with fast-reacting systems, present similar problems, albeit for different reasons. The most noteworthy problem with slow-reacting pre-polymer systems is the requirement for a catalyst.
By introducing a catalyst into the system, processing problems similar to those associated with fast-reacting pre-polymer systems are virtually inevitable. As is well known in the art, the use of a catalyst can severely restrict the ability to control the speed of the reaction, which is undesirable.
Along with the use of a large center, it has now also been discovered that a blend of diamine curing agents with slow reacting prepolymer systems eliminates the problems associated with catalysts while maintaining the advantages associated with slow-reacting prepolymer systems. This cover material has demonstrated superior amalgamation properties when combined with the materials used for the large center. It has been discovered that the combination of this cover material with the large, heavy center and, of course, reduced rubber windings provides a golf ball having all the desirable characteristics of two-piece and three-piece balls without any of the drawbacks associated with the two different types of golf balls.
Accordingly, it is an object of this invention to provide a golf ball that exhibits all the combined desirable characteristics of two-piece and three-piece golf balls such as superior flight distance, good initial velocity, PGA compression and shot feel and control.
Another object of the invention is to lower the cost of manufacture by reducing the amount of thread material needed to make a regulation size golf ball.
A further object of the invention is to employ a cover material that has superior cut and shear resistance. A still further object of the invention is to provide a cover material that exhibits superior amalgamation properties with the other components of the golf ball to thereby increase durability.